Lumiphoric materials are commonly used with electrically activated emitters to produce a variety of emissions such as colored (e.g., non-white) or white light (e.g., perceived as being white or near-white). Such emitters may include any device capable of producing visible or near visible (e.g., from infrared to ultraviolet) wavelength radiation including, but not limited to, xenon lamps, mercury lamps, sodium lamps, incandescent lamps, and solid state emitters—including light emitting diodes (LEDs), organic light emitting diodes (OLEDs), and lasers. Such emitters may have associated filters that alter the color of the light and/or include lumiphoric materials that absorb a portion of a first peak wavelength emitted by the emitter and re-emit the light at a second peak wavelength different from the first peak wavelength. Phosphors, scintillators, and lumiphoric inks are common lumiphoric materials.
LEDs are solid state electrically activated emitters that convert electric energy to light, and generally include one or more active layers of semiconductor material sandwiched between oppositely doped layers. When bias is applied across doped layers, holes and electrons are injected into one or more active layers, where they recombine to generate light that is emitted from the device. Laser diodes are solid state emitters that operate according to similar principles.
Solid state emitters may be utilized to provide colored or white light. White LED emitters have been investigated as potential replacements for white incandescent lamps. A representative example of a white LED lamp includes a package of a blue LED chip (e.g., made of InGaN and/or GaN) combined with a lumiphoric material such as a phosphor (typically YAG:Ce) that absorbs at least a portion of the blue light (first wavelength) and re-emits yellow light (second wavelength), with the combined yellow and blue emissions providing light that is perceived as white or near-white in character. If the combined yellow and blue light is perceived as yellow or green, it can be referred to as ‘blue shifted yellow’ (“BSY”) light or ‘blue shifted green’ (“BSG”) light. Addition of red spectral output from an emitter or lumiphoric material may be used to increase the warmth of the aggregated light output. As an alternative to phosphor-based white LEDs, combined emission of red, blue, and green emitters and/or lumiphoric materials may also be perceived as white or near-white in character. Another approach for producing white light is to stimulate phosphors or dyes of multiple colors with a violet or ultraviolet LED source.
Many modern lighting applications require high power emitters to provide a desired level of brightness. High power emitters can draw large currents, thereby generating significant amounts of heat. Conventional binding media used to deposit lumiphoric materials such as phosphors onto emitter surfaces typically degrade and change (e.g., darken) in color with exposure to intense heat. Degradation of the medium binding a phosphor to an emitter surface shortens the life of the emitter structure. When the binding medium darkens as a result of intense heat, the change in color has the potential to alter its light transmission characteristics, thereby resulting in a non-optimal emission spectrum. Limitations associated with binding a phosphor to an emitter surface generally restrict the total amount of radiance that can be applied to a phosphor.
In order to increase reliability and prolong useful service life of a lighting device including a lumiphoric material, the lumiphoric material may be physically separated from an electrically activated emitter. Separation of the phosphor element permits the electrically activated emitter to be driven with higher current and thereby produce a higher radiance. Structures that separate phosphors from electrically activated emitters create additional problems, however, including (but not limited to) a reduction in total emission resulting from loss of light through the edges of such structures and/or misguided reflection (e.g., total internal reflection (“TIR”)) internal to the structure—such as back upon the electrically activated emitter. Leakage of emissions from an electrically activated emitter past a phosphor can also reduce color uniformity and color rendering. For example, leakage of blue LED emissions past a spatially segregated yellow phosphor can cause aggregate emissions from the device to be perceived (in at least certain directions) as blue shifted yellow or blue shifted green rather than predominately white in character. Any decrease in the amount of light received by the phosphor or other lumiphoric material results in a reduction in light available for upconversion.
U.S. Pat. No. 7,070,300 to Harbers et al. (“Harbers”) discloses a phosphor layer that is physically separated from a light source, permitting the light source to be driven with an increased current to produce a higher radiance. Harbers discloses (e.g., in conjunction with FIG. 1 thereof) a LED and phosphor element oriented at ninety degrees with respect to each other, wherein the phosphor element in one embodiment is separated along the beam path by, e.g., air, gas, or a vacuum, at a length of greater than 1 mm from the LED. Similarly, various elements are represented by Harbers (e.g., in conjunction with FIG. 13 thereof) as being separated from one another, e.g., by an air gap. Such separation of elements and gaps create areas prone to leakage of emissions.
In consequence, the art continues to seek improvements in light emitting structures that include many of the advantages associated with use of remote lumiphoric materials (e.g., minimizing heat degradation), but also limit total internal reflectivity and loss of light that tend to reduce emissions and/or affect perception of output color.